Method of adhering an electrophoretically deposited metal coating to a metal substrate



July 5, 1966 w. SALT T 3,259,562

METHOD OF ADHERING AN ELECTROPHORETICALLY DEPOSITED METAL COATING TO AMETAL SUBSTRATE Filed Oct. 12, 1964 4 Sheets-Sheet l \NVENTORS F.W. SALTBY K.G. LEwls wwmmv BAH-Lois ATTORNEY y 1966 F. w. SALT ETAL 3,259,562

METHOD OF ADHERING AN ELECTROPHORETICALLY DEPOSITED METAL COATING TO AMETAL SUBSTRATE Filed Oct. 12, 1964 4 Sheets-Sheet 2 ATTORNEYS 'July 5,1966 F. w. SALT ETAL 3,2 9,562

METHOD OF ADHERING AN ELECTROPHORETICALLY DEPOSITED METAL COATING TO AMETAL SUBSTRATE Filed Oct. 12, 1.964 4 Sheets-Sheet 5 lNvEN-roRs F.W.SALT BY K 6. Laws k k Nukmh am swlw ls ATTORNEYS METAL COATING TO AMETAL SUBSTRATE 4 Sheets-Sheet 4 Filed Oct. 12, 1964 828mm zGxE $3 3 3 MM 4 r k Q N w W w i w w G. k 5 a E R 1 m o? w w l w SY 3O m 9 UnitedStates Patent 3,259,562 METHOD OF ADHERING AN ELECTROPHORETI- CALLYDEPOSITED METAL COATING TO A METAL SUBSTRATE Frederick William Salt,Swansea, Glamorgan, and Kenneth Gill Lewis, Port Talbot, Glamorgan,Wales, assignors to The British Iron and Steel Research AssociationFiled Oct. 12, 1964, Ser. No. 403,408 Claims priority, application GreatBritain, May 22, 1959, 17,501/59 Claims. (Cl. 204-181) This applicationis a continuation-in-part of our prior application Serial No. 30,316,filed May '19, 1960, now abandoned.

This invention is concerned with a method of forming a metal coating ona metallic substrate and, more particularly, on metallic substrates inelongated form, that is a form which can be subjected to rolling, suchas sheet, strip, wire or rod.

By coating a base metal with another metal, it is frequently possible toobtain an advantageous combination of properties, for example varioustypes of steel can be coated with metals which have a better corrosionand/ or heat resistance than steel, while .the composite materialretains the advantageous mechanical and metallurgical properties of thesteel. Where such a combination of properties can be obtained by theaddition of alloying elements to the base steel, it is often the casethat the alloy steel is more expensive to produce than a coated steelhaving the same properties.

Among the coating metals which can impart useful properties to steelsubstrates are, for example, aluminium, nickel, zinc, copper, brass,stainless steel, cadmium, titanium and zirconium.

Aluminium coated steel is a very useful material for applications wherestrength coupled with corrosion and heat resistant properties arerequired. The self-healing oxide film of the aluminium ensures goodresistance to corrosion and this coating, under certain conditions,particularly in sulphurous industrial atmospheres, provides superiorprotection to that obtained with a zinc coating. The oxide film togetherwith the iron-aluminum alloy formed on heating, provide excellentresistance to scaling at high temperatures. The composite material thusprovides a perfectly satisfactory substitute for an expensive alloysteel in some applications.

Aluminium may be applied to steel by a variety of methods, the principalmethods available being spraying, cladding, vacuum deposition, vapourplating, electro deposition or hot dipping. The processes of sprayingand cladding are well known, the former resulting in a rather porousdeposit lacking in ductility; vacuum deposition has been used to produceextremely thin and highly refleeting surfaces for particularapplications, and thermal deposition of such compounds as tri-isobutylaluminium has been used to produce deposits 0.005 inch thick. It is notpossible to deposit aluminium electrolytically from an aqueous bath andalthough various organic plating baths have been described, this methodis not economic on an industrial scale.

Hot-dipping processes are used on a commercial scale; hot dippedaluminium-coated steel, by reason of the temperature involved in itsproduction, always has a layer of iron-aluminium alloy at the interfacebetween the steel substrate and the coating and while this layerprovides 3,259,562 Patented July 5, 1966 the composite material withgood heat resistance, its brittleness limits the useful applications ofthe material, when, for example it is desired to take advantage of thealuminium coating mainly for its corrosion resistance properties.Cladding essentially involves rolling an aluminium foil onto the steelsubstrate in a plurality of passes so as to obtain a heavy reduction,this being required to obtain good bonding between the substrate and thecoating metal.

Such heavy reduction hardens the substrate and makes it unformable andif a formable material is required, the composite material must then beannealed and this may give rise to the formation of an interlaye'r ofiron-aluminium alloy as mentioned above.

Of the various coating metals other than aluminium mentioned above, onlynickel and copper can be conveniently deposited by electrolysis. In thecase of the other coating metals mentioned, the only methods which aresuitable for the production of tonnage quantities of coated materialhaving coatings some thousandths of an inch thick are cladding andhot-clipping. Zinc and cadmium coatings can be .formed on steel bybot-dipping and brass, stainless steel, titanium and zirconium coatingscan be formed on steel by cladding. In the case of these coating metals,cladding gives rise to the same disadvantages as already mentioned inthe case of cladding with aluminium, that is heavy reduction is requiredwhich substantially reduces the formability of the substrate. Claddingis also subject to the inherent disadvantage that it necessitates theuse of relatively thin toils of the coating metal which may beconsiderably more expensive than the same metal in ingot or powder form.All hot-dipping processes have the inherent disadvantages that aconsiderable heat input is required to maintain the bath of coatingmetal molten and that the molten metal is subject to drossing.

it is an object of the present invention to provide an improved processfor the coating of metallic substrates with other metals. It is afurther object of the present invention to provide a process for coatingmetallic substrates with metals which cannot be economically depositedby electrolysis. It is a still further object of the present inventionto provide a method of torming a metal coating on a metallic substratewhereby a composite material is obtained which can be subjected tosevere deformation without damaging or delamin-ating the coating.Further objects and advantages of the present invention will appear fromthe following description.

Metal coatings are formed, according to the present invention, by aprocess which essentially comprises the steps of electrophoreticallydepositing finely divided coating metal on the substrate, heating thecoated substrate so that it is thoroughly dried and raised to .atemperature adapted to promote adhesion of the coating to the substrate,rolling the coated substrate while still hot with a pressure suflicientto bring the coating metal particles into intimate contact with oneanother, and then heating the coated substrate to effect sintering ofthe coating metal until, but only until, the coating becomes so firmlybonded to the substrate as to prevent delamination upon subsequentdeformation of the coated substrate.

It is known to use the phenomenon of electrophoresis to form coatings ofvarious materials on various types of substrate and as a means ofapplying coatings that can;

of a metallic oxide which is subsequently reduced with hydrogen to themetal, the oxide being deposited alone or simultaneously with anon-reducible metal or nonmetallic compound, such as molybdenumdisulphide or silicon carbide. A particular form of the processgenerically described by Shyne et al. is described by Scheible (one ofthe co-authors of the Plating article) in US. Patent 2,982,707. Scheibleis primarily concerned with the use of prolamines, such as zein, asactivators for the electrophoretic deposition; a typical coating processdescribed by Scheible comprises an electrophoretic deposition step (inthe presence of a prolamine as just mentioned), air drying to remove theelectrophoresis solvents, placing the coated article in a rubberenvelope, evacuating the envelope to remove air, compressing theenvelope isotatically in a glycerine medium at pressures from 20 to 50tons/sq. inch to density the coating, and then sintering the coating ina hydrogen'atmosphere at temperatures of from lO0-l200 C. for from 1 to3 hours. The high sintering temperatures employed by Scheible wouldcertainly lead to grain coarsening of the substrate metal and severelyreduce the formability of the latter. Since we are concerned witheffecting compaction (i.e. densification) of the coating by rolling, wemeasure our compaction pressures in terms of rolling load, i.e.tons/inch width, and it is not possible to make a direct correlationbetween a pressure in tons/ sq. inch and a rolling pressure as thelatter will depend upon the diameter of the work rolls which determinesthe configuration of the nip. It can be said, however, that thepressures of 25 to 50 tons/ sq. inch mentioned by Scheible are much less(by a factor of 2 or 3) than the rolling loads we have found to benecessary in order to obtain adequate compaction of theelectrophoretically deposited coating.

The processes described by Shyne et al. and Scheible are essentiallysuitable only for the treatment of single articles and, in view of thedensification step and the hydrogen reduction step which must in mostcases be carried out at a very high temperatrue, could not economicallybe used for the coating of substrates in elongated form in tonnagequantities.

A continuous process for the formation of aluminium oxide or titaniumoxide insulating coatings on heater filaments for use in thermionicvalves is described by Thomson in US. Patent No. 2,956,937. This is acontinuous process suitable for the large-scale production of coatedwire in which, following electrophoretic deposition of the coatingmaterial, the coated wire is first dried at a relatively low temperatureand is then heated to a much higher temperature to effect sintering ofthe oxide coating, a temperature of from 1300 C. to 1700 C. beingmentioned as suitable for the sintering treatment. If such a sinteringtemperature were used in the case of an aluminium-coated substrate, thealuminium would be com pletely alloyed with the substrate metal and themechanical properties of the substrate metal would be very deleteriouslyaffected.

As indicated above, the process of the present application isessentially characterised by the use of conditions in theafter-treatment of the coating when it has been formed byelectrophoresis, which are such that the final product can be deformedwithout risk of damaging the coating.

Turning now to the drawings:

FIGURE 1 shows diagrammatically apparatus suitable for carrying out thepresent process on relatively narrow strip substrates, that is having awidth of, say, 5 inches;

FIGURES 2 and 3 are photomicrographs of sections through samples ofcoated strip metal which will be described in detail in a subsequentportion of this application; and

FIGURE 4 shows curves of temperature against time for the sintering ofaluminium coatings.

The apparatus comprises an uncoiler for the strip 11, deflector rolls 12and 13, the latter being a contact roll which serves to polarise thestrip as the cathode during operation, a narrow, vertically elongatedelectrophoresis tank 14, the top of which is surrounded by a header 15which collectselectrophoresis suspension flowing over the top edge ofthe tank 14 (which constitutes a weir). The tank 14 contains two sheetanodes (not shown) arranged parallel to and on either side of the pathof the strip. A delivery pipe 16 leads from the header 15 to asuspension reservoir 17 from the bottom of which leads a valved pipe 18which delivers the suspension via a pump 19 to the bottom of theelectrophoresis tank 14. Located vertically above the tank 14 is a pairof inclined rolls 20 which are arranged to deflect the strip and pass itdownwards through a drying duct 21, the rolls 20 being so arranged thatthey contact only the edges of the strip. On leaving the duct 21, thestrip is passed under a further pair of inclined deflecting rolls 22 andinto a pre-heating furnace 23 from which the strip passes directly to acompacting mill 24 and from the latter to a coiler 25.

By using the inclined deflector rolls 20 and 22 and by appropriatedesign of the drying duct 21 and the preheating furnace 23, it isensured that nothing contacts the deposits on the surface of the stripbetween the latter leaving the electrophoresis tank 14 and entering thenip of the compacting mill 24. The lower end of the tank 14 is providedwith a suitable seal or gland to permit entry of the strip whilepreventing egress of the electrophoresis suspension. The drying duct 21and the preheating furnace 23 are provided with heaters capable ofraising the coated strip to the desired temperatures (which are morefully described below); any conventional heat ing means can be used forthis purpose. The pie-heating furnace 24 is also suitably provided withmeans for maintaining a reducing atmosphere therein.

The apparatus illustrated and described above is suitable, as stated,for the coating of relatively narrow strip and for the case where thefinal heat treatment is carried out in coil. If the final heat treatmentis to be carried out continuously, that is, in line, the strip is passedfrom the compacting mill 24 to a sintering furnace arranged in line withthe mill and from the latter to the coiler. The use of inclineddeflector rolls is not practicable with wide strip, and in plant for thecoating of the latter, a suitable arrangement is for a drying andpre-heating furnace to be located vertically above the tank 14 and forthe compacting mill to be located vertically above the pre-heatingfurnace. Once the powder coating has been thoroughly dried, the coatedstrip can, in fact, be passed over a suitably surfaced roll withoutdamaging the coating, chromium plated and tetrafiuoroethylene surfacedrolls being suitable for this purpose. By using such a suitably surfacedroll as a deflector roll vertically above the drying and pre-heatingfurnace in the arrangement just described, the compacting mill can belocated at ground level instead of vertically above the furnace asdescribed, and this arrangement may be considerably more convenient.

The electrophoretic deposition step of the present process can becarried out in a generally similar manner to electrophoretic depositionprocesses previously described. Electrophoresis can be effected in apolar organic medium, suitable organic liquids being, for example,methyl alcohol, ethyl alcohol, for example in the form of industrialmethylated spirit (denatured ethyl alcohol), isopropanol and acetone.The choice of organic solvent is determined by the requirements of lowviscosity, low electrical conductivity, low evaporation rate and highdielectric constant and, of course, the consideration of cost. Withthese considerations in mind, other suitable organic liquids will beapparent to those skilled in the art.

While a wholly organic medium can be used, it is much preferred to use amedium consisting of a mixture of a major proportion of one or moreorganic liquids and a minor proportion of water. By using a partiallyaqueous bath, the electrophoresis yield in terms of grams deposited percoulomb, is considerably increased as compared with deposition underidentical conditions but using a wholly organic medium. Suitableproportions of water are from 0.2% to 30% by volume of the mixture, from2 to 20% being preferred. Within these ranges, it is found that optimumdeposits of different coating metals are obtained with differentproportions of water in the bath. Thus it is preferred to use a bathcontaining v./v. of water when depositing nickel, and a bath containing2% v./v. of water when depositing aluminium.

When using methanol or mixtures of ethanol and methanol as the organiccomponent of the medium, the lower range of water content is best. Ithas been noticed that with higher Water content (say there ispreferential deposition of powder fines. The methanol can in somerespects be considered as replacing the water, since it serves toincrease conductivity. The advantage of such systems is only in reducingcorrosion of the coating metal in the bath. Against this is the factthat there is a tendency for gassing to occur at the substrate surface.

The use of a partially aqueous medium for electrophoresis does result insome degree of gassing occurring at the electrodes, but we have foundthat such gassing is not necessarily deleterious to the coating. Underany given electrophoresis conditions, if the cathode current density(that is the current density at the metallic surface which is to becoated) is progressively increased, there will come a point at which thecoating is deletoriously effected, i.e. by being dislodged, by theevolution of gas, but below this current density value entirelyacceptable coatings are obtained even though some degree of gassingtakes place and the use of water does enable a much greater yield to beobtained as stated above. The limiting current density for the formationof acceptable coatings is dependent upon other conditions of theelectrophoresis and, more particularly, upon the relative velocitybetween the suspension and the metallic surface. The lower this relativevelocity, the lower the limiting current density for the formation ofacceptable coatings, and vice versa. In practice this means that if theprocess is carried out discontinuously, that is the metallic surface isheld stationary within the bath and the latter is agitated justsufficiently to maintain the coating metal particles in suspension, themaximum current density which can be used is about 2 amps/ square foot.One the other hand if the process is carried out by passing theelongated substrate continuously through the bath and the latter isitself circulated continuously through the tank, for example by beingtaken off at the top of the tank and being pumped up from the bottom,current densities greatly in excess of 2 amps/ square foot can be used.For example with a strip speed of 60 feet/ minute and with thesuspension pumped at such a rate that the liquid velocity in theelectrophoresis cell is 75 feet/minute, a current density of 7 amps/square foot can be used, and a coating 0.001 inch thick can be depositedin 5 seconds. It will be appreciated that at higher strip speeds andhigher pumping rates, greater current densities can be used and that theprocess according to the invention is, therefore, suited to the highspeed continuous production in tonnage quantities of metal coatedmetallic strip and wire, such as aluminium coated steel strip and wire,which could not previously be produced economically by the methodsheretofore available. More particularly while the electrophoreticmethods previously described for the formation of meta-l coatings onmetallic substrates were suitable for the coating of single objects bydiscontinuous treatment, they could not be applied to a high speedcontinuous process because the electrophoresis yield was too low.

Suitable panticle sizes for the coating metal in the suspension are upto 50 microns in diameter, the upper limit of size being governed by thedifiiculty of maintaining particles having a diameter greater than 50microns in suspension. Aluminium powder having particle sizes in therange 5 to 50 microns has been successfully used.

The amount of finely divided coated metal in the electrophoresis bath isnot critical, but since the electrophoresis yield has been found toincrease approximately linearly with increasing powder content, it ispreferred to use as high a powder content as is compatible withobtaining sufiicient wet adhesion for the powder coating to withstandthe vibration of the compacting roll. Suitable proportions are, forexample, from 10 to 30% by weight.

The electrophoresis suspension should also contain a soluble salt of amultivalent metal in order to give good electrophoresis yields (i.e. interms of weight of deposit per coulomb) and to provide adequate adhesionof the wet powder to the substrate to allow rapid removal from the bathand of the dried powder to permit in-line rolling; rolling in-line isnot practical unless the powder stays in position despite inevitablerolling mill vibraton. A variety of multivalent metal salts have beenproved to be satisfactory in this respect, for example, salts of nickel,aluminum, manganese, magnesium, zinc, chromium, iron and calcium withmonovalent anions, such as nitrate, chloride, iodide or bromide, may beused. All multivalent metals which can form insoluble hydroxides can berecommended as giving good physical properties after heat treatment.There is usually a residual contaminant in the coating from theelectrolyte and some electrolytes would be better than others inapplications involving the long term corrosion resistance of theproduct. The minimum amount of multivalent metal salt which should beused is that which will just give an adherent continuous coating fromthe bath at maximum line speed and pumping velocity of the suspension.The maximum amount is determined by the fact that above a certainconcentration there is a reduction in electrolysis yield. Providing theelectrolyte concentration is sufficient to produce a uniform layer fromthe bath, the amount and nature of the electrolyte is not importantsince it has been shown that a coating with good wet adhesion also hasgood dry adhesion and rolls well. In general a concentration ofelectrolyte of about 1 millimole per litre has been found to be verysatisfactory.

Following deposition of the coating, the coated substrate is subjectedto a drying and pre-heating step which should be effected in such a Waythat the substrate is not contacted by any roll or guide prior tocompletion of this step since the powder coating would otherwise bedislodged to an undesirable extent. Preferably this is effected bypassing the coated elongated substrate upwards from the electrophoresisbath through a heating zone, provided for example, with radiant electricheaters, which is located in the path of the upwardly moving substrate,there being no contact between the coated substrate and any part of theheating apparatus.

Particularly at high speeds of withdrawal of the coated substrate fromthe electrophoresis bath, dragout of the electrophoresis suspensionoccurs and it is found that the effect of this can be largelyneutralised (and therefore the amount of electrophoresis mediumrequiring removal by evaporation in the heating zone can be considerablyreduced) by subjecting the coated substrate, as it leaves theelectrophoresis bath, to the action of air knives, that is jets of airor other gas directed onto the coated substrate and having a componentof velocity in the direction opposite to that of substrate movement. Theeffect of such air knives is to blow a proportion of the dragged outliquid back into the electrophoresis bath.

The heating effected in the pre-heating step should in all cases be morethan that required to dry the coated substrate thoroughly and we havefound that the temperature to which the coated substrate is raised inthis step has a profound effect upon the ultimate adhesion of thecoating to the substrate. Suitable pre-heating temperatures depend uponthe nature of the coating metal and the substrate metal. In the case ofsteel substrates, suitable pre-heating temperatures are as'follows: foraluminium coatings, from 200 to 350 C.; and for the other coating metalsfrom 100 to 350 C. In the case of these other coating metals,pre-hea-ting should be carried out in a reducing atmosphere, for examplehydrogen or the mixture of nitrogen and hydrogen obtained by crackingammonia, if a temperature above 200 C. is used, but in the case ofaluminium, air containing oxygen is preferred. If the minimumpre-heating temperatures indicated above are not obtained, the coatingof the final product will, in general, have inferior adhesion, eventhough the subsequent steps of rolling and sintering are carried outunder optimum conditions.

The pre-heated coated substrate is then subjected to rolling. Thepurpose of this rolling step is to bring the individual particles of thepowder coating into intimate contact with one another and for anyparticular substrate and coating metal, a suitable minimum rolling loadis relatively critical, in that unsatisfactory coatings are obtained inthe final product if the rolling load is too low. When adhesion of anelectrophoretically deposited coating to the substrate is obtained byheating alone (as taught, for example, by Thomson, US, Patent No.2,956,937), the heat treatment must be longer or at a higher temperatureor both, to achieve a coherent and adherent coating. On the other hand,we have found that when adhesion is secured by compaction due torolling, followed by sintering, the heat treatment can be very much lesssevere and alloying with the substrate, such as steel, takes place to afar smaller extent. In the case of the aluminium coatings with which thepresent invention is concerned, any attempt to replace the minimumcompaction treatment wholly or partly by heat treatment leads in mostcases to the final coating being porous and the heat treatment wouldcertainly have to be so prolonged that brittle alloy would be formedbetween the substrate metal and the coating metal.

The minimum rolling load required to obtain satisfactory compactiondepends upon the hardness of the coating metal and of the substratemetal, and to a lesser extent, as will be more fully described below, onthe roll diameter. The greater the hardness of the substrate, the higherwill be the rolling load required. As a result of numerous experiments,we have developed a formula for determining the minimum rolling load intons per inch width required to obtain adequate compaction; this formulais as follows:

Where R=minimum rolling load in tons per inch width for compaction;

B Brinell hardness of bulk coating metal in the annealed state; and

B =Brinell hardness of the substrate.

The Brinell hardness numbers of some of the coating metals which can beused in the process of the present invention, these figures being forthe metal in bulk form and in the annealed state, are as follows:

Aluminium Copper 42 Brass 70/30 66 Nickel 85 Zinc 3 8 18/8 stainlesssteel 180 The Brinell hardness numbers of some of the substrates whichcan be coated by the process of the present invention, are as follows:

Annealed low carbon steel 82 40% cold reduced low carbon steel 160170The Brinell hardness of other coating metals and other substratematerials can be readily ascertained from the literature by thoseskilled in the art.

In the case of the harder coating metals, that is those having a Brinellhardness of 40 or more in bulk, annealed, form, no variation in therolling load required to obtain adequate compaction is necessary to takeaccount of the diameter of the Work rolls used to effect compaction andfor all work roll diameters, the minimum rolling load given by the aboveformula gives satisfactory results. In the case of the softer coatingmetals, however, that is those having a Brinell hardness of less thanabout 40, the above formula gives the minimum rolling load using 7 inchdiameter work rolls and the rolling load should be increased with largerdiameter work rolls. Thus, for example, if 12 inch diameter work rollsare used in place of 7 inch diameter rolls, the rolling load given bythe above formula should be increased by about 25%. The choice ofrolling load to obtain adequate compaction of the softer coating metalsusing work rolls of other diameters is within the competence of thoseskilled in the art.

Specific examples of suitable minimum rolling loads are as follows-Annealed and temper rolled low carbon steel strip having a Brinellhardness of 82, 0.025 inch thick, and coated with aluminium:

7 inch diameter work rolls 3-4 tons/ inch width (steel extension 3%). 12inch diameter work rolls 4-5 tons/inch width (steel extension 3%).

Unannealed low carbon steel strip having a Brinell hardness of andcoated with aluminium, where the end uses of the product do not requireductility:

7 inch diameter work rolls 8-10 tons/inch width. 12 inch diameter workrolls 10l2 tons/inch width.

Unannealed low carbon steel strip having a Brinell hardness of 165 andcoated with nickel: 1215 tons/inch width.

In the case of annealed and temper rolled low carbon steel strip 0.025inch thick having a Brinell hardness of 82, the following minimumrolling loads we found to be suitable with the coating metals mentioned:

Copper (Brinell hardness 42) About 6 tons/inch width. Brass (Brinellhardness 66) 810 tons/inch width. Stainless steel (Brinell hardness15-20 tons/inch width.

It may additionally be said that in all cases, the attainment of thenecessary compaction can be observed visually. Under-compacted coatingsdo not have the characteristic appearance of a solid sample of thecoating metal, whereas properly compact coatings do. Thusunder-compacted aluminium coatings exhibit characteristic white powderyflecks and under-compacted nickel coatings exhibit an off-white bloom;similar phenomena are observed with the other coating metals. Rolling ispreferably effected with a load which is just suflicient to prevent theappearance of the visual characteristics of undercompaction.

In the case of all substrates and coating metals, insufiicientcompaction leads to a product with poor corrosion resistance and poorformability, that is the coating will crack and/or delaminate when theproduct is bent or drawn, since the adhesion of the coating is poor andthe relatively high temperatures required to effect sintering of theunder-compacted coating lead to the formation of an alloy interlayer.

Rolling loads in excess of the minima described can be used, but whenrolling loads greatly in excess of the minimum for a particularsubstrate and coating metal are used, blisters are occasionally formedin the product during sintering. The effect of rolling loadssubstantially in excess of the minimum is, in general, to causedeterioration of the mechanical properties of the substrate and, inparticular, to reduce its formability. This is of little or noimportance where the coating metal is such that the conditions oftemperature and time required to sinter it are also such as to effectcomplete or partial annealing of the substrate since, in these cases,the subsequent sintering step restores all or most of the formabilityWhich was lost in the compaction step. In the case of nickel, copper,brass or stainless steel coatings on steel, therefore, rolling loadssubstantially in excess of the minima quoted above can be used, sincethe temperature required to sinter all these coating metals is at orabove that at which steel is annealed. In the case of the use of thesecoating metals on steel, the only reason for limiting the rolling loadis economic, that is to limit the size of the compacting mill and thepower required.

In the case of coating meta-ls which are sintered at temperatures belowthe annealing temperature of the substrate, on the other hand, it ispreferred to use rolling loads which are equal to or slightly above theminimum values mentioned above in order to minimise deleterious changesin the formability of the substrate as far as possible. In the case ofsteel substrates, this applies to the use of, for example, aluminium andzinc as coating metals. Thus, for example, if annealed and temper rolledsteel strip having a Brinell hardness of 82 and coated with aluminium,is rolled with a load of tons/inch width of strip with 12 inch diameterwork rolls, an extension of 10% is obtained and the final product haspoor forming qualities.

The final step of the present process consists of a second heattreatment to eflect sintering of the compacted coating; for any coatingmetal sintering may be carried out at a relatively \low temperature fora relatively long time or at a. relatively high temperature for arelatively short time. In general the least severe conditions compatiblewith obtaining complete sintering should be employed. If the sinteringtemperature chosen is such that the time required is more than, say, 30seconds, the coated substrate is advantageously coiled (if in the formof strip or wire) or stacked (if in the form of sheets) after rollingand sintering effected in the coil or stack as the case may be. If thesintering time is sufiiciently short, on the other hand, sintering canbe carried out in line with the sintering furnace located after thecompacting rolls.

Suitable sintering conditions for aluminium are as follows:

(i) 600 C. for 2-15 seconds followed by air cooling.

(ii) Heating to 450 C. in 4-5 hours, followed by holding at thistemperature for 30 minutes.

(iii) Heating to 300 C. in 2 hours, followed by holding at thistemperature for 10-15 hours and then slow cooling.

We have carried out many experiments to determine suitable sinteringconditions for the case of aluminium coatings on steel and the resultsof these experiments are shown graphically in FIGURE 4 of the drawingswhich shows two curves, AB and AC, obtained by plotting sinteringtemperature, in C., against log sintering time in seconds. Anycombination of temperature and time lying between the curves AB and ACgives a satisfactory, i.e. tightly adherent, coating, whereas anycombination of temperature and time lying above the curve AB will givean unsatisfactory coating due to the formation of a deleterious quantityof brittle alloy between the coating and substrate metals, and anycombination of temperature and time lying below the curve AC will alsogive an unsatisfactory coating due to the aluminium being unsintered orinsufiiciently sintered. Suitable sintering conditions for aluminiumcoatings on steel can, therefore, be readily read off from FIGURE 4.

Suitable sintering conditions for a variety of other coating metals areas follows:

Nickel:

(i) 1000 C. for 2 seconds followed by cooling in static gas. (ii) 700 C.for 5 hours. (iii) Heating to 680 C. in 3 hours and holding for 6 hours.Copper: 600700 C. for 30 minutes. Brass: 600700 C. for 30 minutes.Stainless steel: 10001100 C. for 60 minutes. Aluminum containing 1%, 5%or 10% zinc: temperature up to 400 C.

The following examples of the present process are given by way ofillustration and not limitation:

Example I Annealed and temper rolled steel strip was coated withaluminium in apparatus of the kind illustrated in FIG URE 1.

The strip had a width of 5 inches and thickness of 0.025 inch and hadbeen degreased by cathodic treatment in hot alkali, lightly pickled incold 50% v./v. hydrochloric acid, and then washed. The strip was passedcontinuou'sly into the electrophoresis cell through the bottom of thecell and passed vertically upwards through the bath contained therein,the path length of the strip in the bath being 30 inches. The bath wascontinuously circulated through the cell, being pumped in at the bottomof the cell and allowed to flow over a weir at the top before beingrecirculated to the pump. A steel anode 30 inches long and 5 inches widewas arranged parallel to each side of the strip, the spacing between thesurface of the strip, which was connected as the cathode, and thesurface of the anodes being 1 inch.

The bath consisted of a 10% W./v. suspension of aluminium powder ofwhich passed through a 300 mesh sieve) in a mixture of 80% by weight ofindustrial methylated spirit and 20% by weight of water and contained1.0 millimole/ litre of aluminium nitrate.

The strip speed was 10 feet/minute, the bath velocity in the cell was 75feet/ minute, the applied voltage 75 volts and the cathode currentdensity 2.4 amps./ square foot.

After passing out of the electrophoresis cell, the coated strip wasdried by passing it through an electrically heated drying chamber whichraised the temperature of the coated strip to 200 C. and then passedthrough a rolling mill having 7 inch diameter work rolls and giving aload of 4 tons/inch width of strip. The thickness of the coating afterrolling was 0.001 inch.

The rolled strip was then coiled and heated in the coil to 500 C. over aperiod of 3 hours, the rate of heating being such that its temperaturewas below 300 for 2 hours. The coiled strip was then allowed to cool inair.

The resulting coating was tightly bonded to the strip and the coatedstrip could be bent round a mandrel having a diameter equal to thethickness of the coated strip without delamination occurring.

Example 2 Coating of annealed and temper rolled mild steel strip similarto that of Example 1 was carried out substantially as described in thatexample, except that the anodes were formed of pure nickel and were 36inches long, their spacing from the strip being as before.

The suspension contained 10% w./v. of aluminium powder having a particlesize range of 5 to 50 microns in a mixture of 98% by weight ofindustrial methylated spirit and 2% by weight of water which alsocontained 1 millimole/ litre of nickel chloride.

The strip was passed through the electrophoresis cell at 10 feet/minute,a current of 1.5 amps was applied to each side of the strip and theapplied voltage was volts; the finished thickness of the coating was 25microns.

In a number of runs, the coated strip was subjected to differentcombinations of after-treatments, rolling being effected in each casewith 12 inch diameter work rolls. The results obtained are summarisedbelow; in these results, satisfactory coating means that the coatedstrip The suspension consisted of 30% by weight of nickel powder in amixture of 90% by volume of industrial methylated spirit and 10% byvolume of water which also contained 1 millimole of nickel chloride perlitre.

The strip was passed through the electrophoresis cell could be bentround a inch mandrel without damagat feet/mlnute, a current of 1.5 ampswas applied to ing or delamlnatmg the coating and unsatisfactorycoateach side of the strip and the applied voltage was 60 mg means thatthe coating was damaged and/ or devolts. The finished thickness of thecoating was laminated on being sub ected to this test. microns.

Pre-heating Rolling load, sintering temperature, Run temperatons/in. 0.Product ture, 0. width 250 4 450 in 4-5 hours and Satisfactory coating(see hold for minutes. Figure 2). 350 5 do Satisfactory coating. 250 10300 for 15 hours Unsatisfactory coating (overcompacted). 250 2 450 in4-5 hours and Unsatisfactory coating hold for 30 minutes.(undereompactcd). 4 600 in 15 seconds Unsatisfactory coating.

250 4 u-.. o Satisfactory coating. 250 4 520 for2hours... Unsatisfactorycoating (brittle alloy). 250 4 620 for 10 minutes Unsatisfactory coating(see Figure 3 (a)). 250 4 620 for 15 minutes Unsatisfactory coating (seeFigure 3 (12)). 250 4 620 for 60 minutes Unsatisfactory coating (seeFigure 3 (0)).

1 Omitted, coating dried at 60 0.

FIGURE 2 of the accompanying drawings comprises photomicrographs at amagnification of X500 of a section through (a) an undistorted sample ofthe product obtained in run 2a and (b) a sample of the same productwhich had been bent round a inch mandrel, the latter photomicrographbeing of a portion of the sample near the apex of the bend. In boththese photomicrographs 26 is the steel substrate and 27 the compactedand sintered aluminium coating. It will be seen from. FIGURE In a numberof runs, the coated strip was subjected to different combinations ofafter-treatments, rolling being effected in each case with 12 inchdiameter work rolls. The results obtained are summarised below; in theseresults, satisfactory coating means that the coated strip could be bentaround a inch mandrel without damaging or delaminating the coating andunsatisfactory coating means that the coating was damaged and/ordelaminated on being subjected to this test.

1 Omitted, coating simple dried at 60 0.

thicker from FIGURE 3(a) to FIGURE 3(a), showing the increasingformation of this alloy on holding at a high sintering temperature. Theproducts of runs 211, 2i and 2j all failed the deformation test becauseof the presence of the layer of brittle iron-aluminium alloy shown inFIGURE 3, the formation of which was avoided by the process followedwith runs 2a, 2b and 2f.

Example 3 Coating of hard unannealed mild steel strip 5 inches wide and0.025 inch thick with nickel was carried out in apparatus as describedin Example 2.

2 Reducing atmosphere.

Example 4 Steel strip of the type used in Example 1 was coated with zincby a procedure substantially as described in that example, except thatthe aluminium powder in the suspension was replaced by zinc powder (10%w./v.) and the aluminium nitrate electrolyte was replaced by Zincnitrate (1 millirnole per litre).

On leaving the electrophoresis cell, the coated substrate was heated to200 0., passed through a rolling mill having 7 inch diameter work rollsand giving a load of 4 tons/inch width of strip and coiled. The coiledstrip was heated for 1 hour at 250 C. to give a tightly adherent zinccoating.

The metallic substrate should, of course, be free of grease and othercontaminants prior to effecting electrophoretic deposition thereon.Suitable metal cleaning procedures are well known to those skilled inthe art and one suitable cleaning procedure has been described in theabove examples. Degreasing by cathodic treatment in hot alkali followedby washing in water is in many cases quite sufficient. A more thoroughcleaning schedule comprises (a) immersion of the substrate in a aqueouscaustic soda solution at 60 C., (b) cathodic electrocleaning in anaqueous solution containing caustic soda and 5% sodium carbonate using acurrent density of 100 amps/square foot, and (c) fiash pickling inhydrochloric acid. In a typical process treatment (a) is effected for 20seconds, and the substrate is then washed in water, treatment (b) iseffected for 30 seconds and the substrate is then washed in water, andtreatment (c) is effected for seconds and the susbtrate is again washedbefore being passed into the electrophoresis tank.

Another suitable cleaning procedure comprises subjecting the substrateto vapour blasting; this gives a clean matt surface.

While in the foregoing exam'ples, we have described application of theprocess to the coating of strip, it is equally applicable to Wire. Inthis case, a hollow-cylindrical anode is preferably employed, the wireto be coated being aligned along its axis. Rolling of the coated wireafter the preheating step is preferably carried out by two passesbetween shaped rolls which are designed so that 60% of the wire surfaceis compacted at each pass, the axes of the rolls for the second passbeing at right angles to the axes of the rolls used for the first pass.In this way the coating is uniformly compacted around the wire. Thedegree of pressure applied by the rolls is preferably such as to cause a1% reduction in cross-sectional area in the case of a mild steel wire; asuitable rolling load is, for example, approximately 0.2 ton per standfor hard drawn 01120 inch mild steel wire. In the case of harder, i.e.less deformable, wires the reduction in area can be less, but should beat least 0.2%. The second stand may, if desired, be rotated with respectto the first stand to allow for any twisting of the wire as .a result ofinduced stresses. The rolls should be kept clean, i.e. free from metalpowder, suitably by means of rotating brushes.

Instead of two passes through shaped rolls, two fourroll Turks headrolling units can be used for compacting the powder coating, but thisoffers no advantages over the simpler and cheaper double roll unitsalready described. The powder can also be compacted by rolls describinga helical path around the wire.

Whichever arrangement of rolls is used to compact the coating, they willbe heated by the coated wire which is still hot from the first heattreatment and it may be desirable to provide means for cooling thecompacting rolls.

What is claimed is:

1. The method of electrophoretically depositing a metal coating on ametal substrate in elongated form, which method comprises the step ofcontinuously passing the substrate through an electrophoresis cellcontaining a suspension of finely divided coating metal in its metallicform in a polar organic solvent said suspension also con taining from 2%to 30% water by volume and a minor proportion of a soluble rnultivalentmetal salt, said substrate being connected as cathode and saidsuspension being continuously agitated, whereby said coating metal iselectrophoretically deposited on said substrate.

2. The method claimed in claim 1, in which said metal coating isaluminium, and comprising the additional steps of heating said coatedsubstrate to a temperature between 200 and 350 C., rolling said coatedand heated substrate with a rolling load at least equal to that given bythe formula Where R is the minimum rolling load in tons per inch width,B is the Brinell hardness of the bulk coating metal in the annealedstate, and B is the Brinell hardness of the substrate, and thensintering said coated substrate at a temperature insuflicient to causethe formation of a brittle alloy layer at the interface between thecoating and substrate.

3. The method claimed in claim 1, in which said metal coating isaluminium, and comprising the additional steps 14 of heating said coatedsubstrate to a temperature between 200 and 350 C., rolling said coatedand heated substrate with a rolling load at least equal to that given bythe formula where R is the minimum rolling load in tons per inch width,B is the Brinell hardness of the bulk coating metal in the annealedstate, and B is the Brinell hardness of the substrate, and thensintering said coated substrate under conditions of temperature and timeinsufiicient to cause the formation of an alloy layer at the interfacebetween the coating and substrate, said time and temperature lyingbetween the curves AB and AC of FIGURE 4 of the drawings.

4. The method claimed in claim 1 according to which said substrate issteel, the coating metal is selected from the group consisting ofcopper, brass, stainless steel, zinc and nickle, and said watercomprises from 2 to 20% by volume of the bath, said method comprisingthe further steps of continuously passing said coated substrate to astation at which it is heated to a temperature between and 350 C.,rolling'said coated and heated substrate with a rolling load at leastequal to that given by the formula where R is the minimum rolling loadin tons per inch width, B is the Brinell hardness of the bulk coatingmetal in the annealed state, and B is the Brinell hardness of thesubstrate, and then sintering the coated substrate.

5. The method of causing an electrophoretically deposited coating ofaluminium to adhere to a steel substrate sufficiently to preventdelamination when said coated substrate is subsequently bent about a Ainch mandrel, which method comprises the steps of preheating said coatedsubstrate to a temperature between 200 and 350 C., compacting saidcoated substrate while still hot with a rolling load of at least threetons per inch of subtrate width, and then sintering the coated substrateunder conditions of temperature and time insufiicient to cause theformation of an alloy layer at the interface between the coating andsubstrate, said time and temperature lying between curves AB and AC ofFIGURE 4 of the drawings.

6. The method of causing an electrophoretically deposited coating ofaluminium to adhere to a steel wire which method comprises the steps ofpreheating said coated wire to a temperature between 200 and 350 C.,compacting said coating by compressing said heated and coated wire alongat least two diameters positioned at right angles to each othersufficiently to reduce the crosssectional diameter of the wire between0.2% and 1%, and then heating said coated wire to a temperature and fora length of time suflicient to sinter the coating but insufiicient tocause the formation of a layer of alloy at the interface between thecoating and substrate said time and temperature lying between curves ABand AC of FIG. 4 of the drawings.

7. The method of causing an electrophoretically deposited coating ofcopper to adhere to a steel substrate sufficiently to preventdelamination when said coated subtrate is subsequently bent about a inchmandrel which method comprises the steps of preheating said coatedsubstrate to a temperature between 100 C. and 350 C., compacting saidcoated substrate while still hot with a rolling load of about 6 tons perinch of substrate width and then sintering the coated substrate atbetween 600 and 700 C.

8. The method of causing an electrophoretically deposited coating ofbrass to adhere to a steel substrate sufficiently to preventdelamination when said coated substrate is subsequently bent about ainch mandrel which method comprises the steps of preheating said 15coated substrate to a temperature between 100 and 350 C., compactingsaid coated substrate while still hot with a rolling load of from 8 to10 tons per inch of substrate width and then sintering the coatedsubstrate at between 600 and 700 C.

9. The method of causing an electrophoretically deposited coating ofstainless steel to adhere to a steel substrate sufiiciently to preventdelamination when said coated substrate is subsequently bent about ainch mandrel which method comprises the steps of preheating said coatedsubstrate to a temperature between 100 and 350 C., compacting saidcoated substrate while still hot with a rolling load of from 15 to 20tons per inch of substrate width and then sintering the coated substrateat between 1000 and 1100 C.

10. The method of causing an electrophoretically deposited coating ofnickel to adhere to a steel substrate sufiiciently to preventdelamination when said coated substrate is subsequently bent about ainch mandrel which method comprises the steps of preheating said coatedsubstrate to a temperature between 100 and 350 C., compacting saidcoated substrate while still hot with a rolling load of from 12 to 15tons per inch of substrate width and then sintering the coated substrateat no more than 1000 C.

References Cited by the Examiner UNITED STATES PATENTS 2,37 2,607 3 1945Schwarzkopf 75208 2,442,863 6/ 1948 Schneider 204-181 2,878,140 3/1959Barr 204181 2,935,402 5/1960 Trotter et al. 75208 2,982,707 5/1961Scheible 204181 3,142,560 7/1964 Storchheim 75-208 ALLEN B. CURTIS,Examiner.

1. THE METHOD FO ELECTROPHORETICALLY DEPOSITING A METAL COATING ON AMETAL SUBSTRATE IN ELONGATED FORM, WHICH METHOD COMPRISES THE STEP OFCONTINUOUSLY PASSING THE SUBSTRATE THROUGH AN ELECTROPHORESIS CELLCONTAINING A SUSPENSION OF FINELY DIVIDED COATING METAL IN ITS METALLICFORM IN A POLAR ORGANIC SLVENT SAID SUSPENSION ALSO CONTAINING FROM 2%TO 30% WATER BY VOLUME AND A MINOR PORPORTION OF A SOLUBLE MUTIVALENTMETAL SALT, SAID SUBSTRATE BEING CONNECTED TO CATHODE AND SAIDSUSPENSION BEING CONTINUOUSLY AGITATED, WHEREBY SAID COATING METAL ISELECTROPHORETICALLY DEPOSITED ON SAID SUBSTRATE.
 4. THE METHOD CLLAIMEDIN CLAIM 1 ACCORDING TO WHICH SAID SUBSTRATE IS STEEL, THE COATING METALIS SELECTED FROM THE GROUP CONSISTING OF COPPER, BRASS, STAINLESS STEEL,ZINC AND NICKEL, AND SAID WATER COMPRISES FROM 2 TO 20% BY VOLUME OF THEPATH, SAID METHOD COMPRISING THE FURTHER STEPS OF CONTINUOUSLY PASSINGSAID COATED SUBSTRATE STATION AT WHICH IT IS HEATED TO A TEMPERATUREBETWEEN 100 AND 350*C., ROLLING SAID COATED AND HEATED SUBSTRATE WITH AROOLING LOAD AT LEAST EQUAL TO THAT GIVEN BY THE FORMULA